Pulse Width Modulated - Constant Temperature Anemometer (PWM-CTA)

Overview:

The standard hot-wire anemometer functions based on a Wheatstone Bridge with one resistor replaced by the probe wire. By sampling the voltage across the bridge the fluid velocity at the sensor can be determined. The Pule Width Modulated - Constant Temperature Anemometer applies a set voltage until the wire reaches the desired overheat, the duty cycle of this pulse is then sent back to the PC as an integer number. Overall, the PWM-CTA presents advantages in speed, cost and data analysis over the traditional designs.

Benefits:

  • Digital output eliminates the need for additional analog to digital conversion equipment
  • Up to 18 independent channels synchronously sampled
  • Increased sensitivity compared to a conventional CTA. This is due to the second order response of the PWM-CTA vs. fourth order response of a standard CTA.
  • Frequency response can be made proportional to velocity
  • PWM-CTA control system is inherently stable
  • Relative cost per channel for the PWM-CTA is significantly lower than an analog CTA with similar capabilities.

Features:

  • Windows graphical user interface
  • Synchronous sampling on up to 18 channels.
  • All channels independently configurable
  • Trigger in and trigger out for synchronization with other measurement devices
  • Sample frequency of 100kHz, 50kHz 33.3kHz
  • User Configurable channel operation - all channels can be configured and run independently.
    • Pulse Width Modulation for driving a hotwire. Allows gain-bandwidth, slew rate, and gain adjustments for noise elimination
    • Resistance measurement 0 - 10 Ohms for cold resistance of hotwires
    • Analog voltage measurement for synchronously sampling from other devices. User adjustable input voltage range, ±10V, ±5V, ±2.5V, ±1.25V

Disclosure:

The currently available electronic implementation of the PWM-CTA exhibits a modest attenuation in its output for frequencies ≥ 3kHz. A representative example ofthis attenuation is shown in the figure below. These data were taken in the sheer layer of a jet: ū = 5.25 m/sec. We are currently working to increase this range.

PWM-CTA high frequency attenuation

Pictures:

Conventional constant temperature anemometer schematic

PWM-CTA functional schematic

Eighteen channel PWM-CTA. Each vertical set of connectors is one channel.

Screenshot of the PWM-CTA computer control interface.